The steam reforming reaction for the production of synthesis gas is well known, viz. EQU CH.sub.4 +H.sub.2 O.revreaction.CO+3H.sub.2 ( 1)
the reaction being endothermic, and occurring at elevated temperatures and pressures, e.g., above about 1500-1600.degree. F. and 25-30 atmospheres, in the presence of a suitable catalyst usually supported nickel catalyst. Another method for producing synthesis gas is the non-catalytic partial oxidation reaction which is exothermic and illustrated as: EQU CH.sub.4 +O.sub.2 .fwdarw.CO+H2+H.sub.2 O (2)
These reactions have been combined in various prior references to produce synthesis gas by the reaction of a light hydrocarbon, e.g., methane, with steam and oxygen in the presence of a suitable catalyst in either fixed or fluid beds. The combined reaction is EQU CH.sub.4 +1/20.sub.2 .fwdarw.CO+2H2 (3)
which results in a synthesis gas product that can be directly used in the synthesis of higher hydrocarbons via the Fischer-Tropsch process where the stoichiometric requirement for hydrogen to carbon monoxide is about 2/1.
Fluid bed processes are particularly useful in that excellent mixing is effected along with high heat transfer to prevent reaction runaway and hot spots. Fluid beds are, however, a mixed blessing in that the constant motion of the catalyst particles, against other particles and reactor walls, causes catalyst attrition and entrainment of fine catalyst particles, e.g., less than about 20 micron, in the overhead product gas, no matter the efficiency or number of catalyst recovery cyclones. These particles tend to deposit in equipment lines downstream of the fluid bed reactor where product gas cooling occurs and as the temperature decreases, these particles can promote the back reaction of carbon monoxide and hydrogen to re-form methane, resulting in serious product debits. Note that reaction (1) is reversible. In addition, as the product gas is cooled, there is a tendency for carbon to form as a result of the well known Boudouard reaction EQU 2CO.fwdarw.C+CO2 (4)
Carbon deposition in equipment leads to clogging of lines and premature process turnarounds to clear those lines.
An object of this invention is to provide a synthesis gas preparation process--including both the reaction and the cooling steps--that substantially preserves, as carbon monoxide and hydrogen, the hydrocarbon conversion to synthesis gas in the reaction zone. Another object is to prevent carbon deposition in equipment lines during the cooling step. These and other objects are accomplished by the practice of this invention.
Another object of this invention is to make efficient use of the heat recovered in the cooling zone and to save energy by using that heat, in the form of high pressure steam, to preheat the light hydrocarbon feed to the reaction zone.
In a reaction zone, a light hydrocarbon feed, primarily methane as in natural gas, is reacted with oxygen and steam in the presence of a supported nickel catalyst. The reaction zone is maintained at elevated temperatures and pressures and a reaction product comprising synthesis gas (carbon monoxide and hydrogen) and some entrained catalyst particles is produced. The conversion of the feed to synthesis gas, carbon monoxide and hydrogen, is maintained in the subsequent cooling zone by rapidly cooling the product gas from the reaction zone by indirect heat exchange to a temperature below which the back reaction of carbon monoxide and hydrogen to form methane is favored. The rapid cooling substantially prevents both the back reaction to form methane in the presence of entrained catalyst and the deposition of carbon in accordance with the Boudouard reaction. Rapid cooling through the use of indirect heat exchange, i.e., shell and tube exchange, generates steam, at least a portion of which is used to preheat the light hydrocarbon feed, thereby saving on the fuel required to heat the feed to reaction temperature.
The rapid cooling that is effected in the cooling zone takes the product from the temperature of the synthesis gas generation reaction in the reaction zone to a temperature below that which favors the back reaction of carbon monoxide and hydrogen to form methane in the presence of entrained catalyst. Thus, the product gas is cooled from temperatures in excess of about 1700.degree. F. to a temperature below about 1200.degree. F.; preferably below about 900.degree. F. In this way, at least about 85% of the feed converted to synthesis gas is maintained as synthesis gas. Further increasing the rate of cooling serves to preserve somewhat more feed conversion as synthesis gas and substantially decreases the amount of carbon formed by the Boudouard reaction. (The net methane conversion is the total of methane and other light hydrocarbons converted to synthesis gas in the reaction zone less the amount of methane formed by the back reaction of carbon monoxide and hydrogen that is promoted by the catalyst carried over from the reaction zone into the cooling zone. The methane conversion preserved is the percent methane remaining after cooling relative to the total conversion of methane and other light hydrocarbons in the feed converted to synthesis gas.)